Found the cause, it was Pro Duo GPUs 3 and 4. As soon as i disabled those two sensors, it stopped. It slowed after disabling the sensor for GPU3, and stopped after 4 was disabled. http://imgur.com/Kke4DPx

Thanks for the results, this is interesting. Now we should somehow get closer to why this happens...
Would you be able to determine if there's some particular sensor readout under those sensors that when disabled will also solve this ? So enable back the entire sensor headings and try to check particular readouts.

Can you please try another test?
If you're using Sensors-only mode, then uncheck that setting. Then run HWiNFO and you should get a summary window. Leave that window opened without opening the sensors window and watch memory usage. In the summary window you can also choose which GPU to show, so if the memory grows you should also know which GPU polling is causing it.

In the process of restoring sensors and trying (unsuccessfully) to restore that welcome screen, the issue has gone away. Not sure why or how, but it looks like memory consumption is peaked at around 100mb. I'll keep testing over the next few days and let you know.